Super sharp, great AF (considering you set the focus distance switch correctly) good color and contrast. Just an incredible piece of glass.

Cons:

If there is a negative it is the price although consider all the elements, IS and build it is questionable how Canon could sell it for less.

I dreamed about owning this lens for a year was finally able to make it happen. It is everything I expected and more. Before I had the tripod setup (Gitzo 1325 & full Wimberley) I shot it handheld on about 4 outings. While heavy it is handholdable. The handheld photos came out very sharp which is a testiment to the AF and the IS. Also shot quite a few with the EF 1.4X II Teleconverter. You cannot tell the difference in image quality with and without this on. I am awaiting a 2X so hopefully the performance with that will be close to the 1.4X.

After buying it I have not had one second of remorse. I can already tell it will give me the confidence to shoot wildlife I normally wouldn't have gone after and to go to places I wouldn't have traveled to find that wildlife.

Very protective of the gear and holds a lot - easily fits 15" Powerbook.
Good carry on for airplane travel.

Cons:

Uncomfortable as a back pack - see comments.
Tripod thing gets in way of getting to gear.

I have traveled to Europe, Mexico, South America with this bag full of my gear and it worked well for that. What it doesn't work for is hiking. I completely threw my back out with this thing after a day of hiking and shooting in Mexico and I have never had a back problem in my life. I could barely walk the next day and it was the day I was leaving. My poor wife had to act like a Sherpa for me as I couldn't even lift a dop kit. It was fully loaded that day however. I have learned to lighten the load by half but then you have to leave behind things you may need on the shoot.